1. Field of the Invention
This invention relates to monomeric adhesive materials and more particularly relates to a new process for producing monomeric 2-cyanoacrylates.
2. Description of the Prior Art
2,4-Dicyanoglutarates (symmetrical diesters of 2,4-dicyanoglutaric acid) are starting materials obtainable by reacting 2 moles of cyanoacetate with 1 mole of formaldehyde for the synthesis of 2-cyanoacrylates, although they are also formed as secondary products in the production of 2-cyanoacrylates. Numerous attempts have made to convert these valuable starting materials (whether specifically produced or obtained as residue in the thermolysis of oligomeric 2-cyanoacrylates in admixture with other substances) into 2-cyanoacrylates.
Thus, it was proposed in the U.S. Pat. No. 2,926,188 to split 2,4-dicyanoglutarates thermally into 2-cyanoacrylates and cyanoacetates. The process is carried out at temperatures in the range of from 175.degree. C. to 210.degree. C. Unfortunately, these high reaction temperatures promote secondary reactions to the detriment of the yield. Because of this, it was proposed in German Patent Application No. 32 33 007, first publicly disclosed on Mar. 8, 1984, to catalyze the thermolysis process by using bases, such as alkali or alkaline earth alcoholates. This makes it possible to carry out the reaction at temperatures in the range of from 90.degree. C. to 130.degree. C. and to obtain 2-cyanoacrylates in an increased yield. Since the boiling points of cyanoacetates and 2-cyanoacrylates differ only slightly in some cases, depending on the alcohol used for esterification, fairly complicated distillation systems have to be used in both processes mentioned above to enable the products to be obtained in the required purity.
In addition, it is known from U.S. Pat. No. 3,254,111 that 2,4-dicyanoglutarates can be condensed with paraformaldehyde in the presence of basic catalysts and that the product thus obtained can be thermally split into 2-cyanoacrylates. The disadvantage of this process lies in the relatively poor yields of 2-cyanoacrylate. If, in addition, an amine, for example piperidine, is used as the condensation catalyst, the products obtained have poor bonding properties. Furthermore, additional expense is involved in removing the benzene used as entraining agent.
Accordingly, there is a need for a new process for producing 2-cyanoacrylates from 2,4-dicyanoglutarates in high yields at relatively low temperatures. In addition, the products must have the purity required for bonding purposes.